1. Field of the Invention
This invention relates to a photosensitive member and more particularly to, for instance, an electrophotographic photosensitive member.
2. Description of the Prior Art
Heretofore, there have been known electrophotographic photosensitive members such as one prepared from Se or Se doped with As, Te, Sb and another prepared from a resin binder with ZnO and CdS dispersed therein. However, the problem with these electrophotographic photosensitive members is that they form a source of environmental pollution and lack both thermal stability and mechanical strength.
There has recently been proposed an electrophotographic photosensitive member using amorphous silicon (a-Si) as a host crystal. Amorphous silicon has a dangling bond, so to speak, wherein the Si--Si bond has been served and, because of this defect, there are a number of localized levels within an energy gap. For this reason, hopping conduction generated in the thermally-excited carrier causes dark resistance to remain small, whereas the photo-excited carrier is trapped at the localized level and reduces photoconductivity. Consequently, hydrogen atoms (H) are used to compensate for the defect by coupling H to Si and cover the dangling bond.
The specific resistance of the hydrogenated amorphous silicon (hereinafter called the "a-Si:H") in dark regions is 10.sup.8 .about.10.sup.9 .OMEGA.-cm, which is roughly 1/10,000 lower than that of amorphous Se. Accordingly, the disadvantage of the photosensitive member comprising a single layer of a-Si:H is that the dark damping rate of the surface potential is large with the initial charging potential being low. The photosensitive layer of the member above has excellent characteristics, because its specific resistance will be reduced to a greater extent if light in visual and infrared regions is applied to the photosensitive member.
FIG. 1 shows an electrophotographic copying machine incorporating the a-Si:H as the base metal of a photosensitive member of a-Si series. The copying machine above comprises a document glass plate 3 for mounting an original 2 and a platen cover 4 for covering the original 2 in the upper portion of a cabinet 1; a light source 5 and a first reflection mirror 6 constituting a first mirror unit 7 as an optical scanner linearly horizontally movable under the glass plate 3; and a second mirror unit 20 for making constant the optical path length between the original scanning point and the photosensitive member, the second mirror unit 20 being moved at a velocity proportional to that of the first mirror unit. The light reflected from the glass plate 3 is made incident on a photosensitive drum 9 as an image retainer in the form of a slit through a lens 21 and a reflection mirror 8. The copying machine further comprises a corona charger 10, a developer 11, a transfer portion 12, a separator 13 and a cleaner 14 arranged around the drum 9, a toner image on the drum 9 being transferred onto copying paper 18 fed from a paper feeder box 15 through paper feeding roller 16, 17 and fixed by a fixing portion 19 before being discharged to a tray 35. In the fixing portion 19, the copying paper carrying a developed image therewith is passed between a heating roller 23 containing a heater 22 and a pressure roller 24 to effect the fixing operation.
However, whether or not the a-Si:H as the surface layer of the photosensitive member is chemically stable has not thoroughly been examined in terms of the influences derived from its long-term exposure to air or moisture and the type chemical of generated by corona discharge. For instance, what has been left untreated for more than one month is seen to be affected by moisture and the receptive potential is greatly reduced. "Phil. Mag. Vol. 35 (1978)", for example, refers to a method for manufacturing, and the existence of, amorphous hydrocarbonized silicon, which has made it clear that silicon is characterized by not only high heat resistance and surface hardness but also by shadow resistivity higher (10.sup.12 .about.10.sup.13 .OMEGA.-cm) than that of a-Si:H and a wide changeable energy gap depending on the quantity of carbon ranging from 1.6 to 2.8 eV. However, a drawback is that long wavelength sensitivity is poor because the band gap increases with the percentage of carbon contained.
Japanese Patent Application Laid-Open Gazzette 55-127,083 discloses such as electrophotographic photosensitive member constructed of a combination of a-SiC:H and a-Si:H, according to which the a-Si:H layer is used as a charge generating (photoconductive)layer and the a-SiC:H as a charge transporting layer provided under the charge generating layer to form a function separating type double layer construction, so that photosensitivity in a wide wavelength range and improved charge potential can be obtained from the upper a-Si:H layer and the lower a-SiC:H layer forming a hetero junction with the former, respectively. However, the dark attenuation of the a-Si:H layer is not sufficiently prevented and the charge potential is still insufficient to the extent that it cannot be put to practical use. Moreover, the presence of the a-Si:H layer on the surface has resulted in poor chemical stability, mechanical strength, heat resistance and the like of the product above.
According to Japanese Patent Application Laid-Open Gazette 57-17,952, the first a-SiC:H layer is formed as a surface modifying layer on a charge generating layer composed of a-Si:H and the second a-SiC:H layer is formed as a charge transporting layer on the under surface thereof (on the support electrode side). As for the above-described photosensitive member of the prior art, although the prevention of dark attenuation and the chemical stability of the surface are expected because of the surface modifying layer, the following problems are seen to have been posed:
That is, the specific resistance (dark resistivity) .rho.D of the a-SiC:H as one forming the surface modifying layer above is limited to and cannot exceed 10.sup.13 .OMEGA.-cm; consequently, the retention of the charge potential is insufficient. Moreover, if SiO.sub.2 is used for the surface improving layer, although .rho.D will be increased, the active elements (ions, molecules and atoms in the discharge atmosphere) generated in the neighborhood of the charged electrode will readily be attracted to the surface and this will cause image flow to be easily produced as a creeping discharge occurs. Although it is true that the active elements such as SiO.sub.2 are hardly attracted to the a-SiC:H, the capability of the charge potential retention (retention capability particularly at high temperature and moisture) since .rho.D is insufficient as described above.